Conventionally, it is known that an electrical discharge state in an electrical discharge machining in an electrical discharge machine can be judged by detecting the magnitude of a high-frequency component of the electrical discharge voltage waveform. This electrical discharge voltage waveform is a quite complex waveform including a high-frequency component. Accordingly, it is quite important to provide a technique in which only a characteristic waveform component is detected from the electrical discharge voltage waveform without fail and at high speed.
Japanese Laid-Open Patent Publication No. 5-293714 discloses an electrical discharge state detecting device for an electrical discharge machine. Referring to FIG. 11, illustrating a circuit of substantially the same constitution as that of this reference, the operation of this device will be described hereunder.
In FIG. 11, reference numeral 2 denotes an electrode of an electrical discharge machine, reference numeral 3 denotes a workpiece, and a machining clearance is formed between the electrode 2 and the workpiece 3. Reference numeral 1 denotes a machining power source of the electrical discharge machining machine. An electrical discharge voltage in the form of a pulse is supplied from the machining power source 1 to the machining clearance between the electrode 2 and the workpiece 3. Reference numeral 4 denotes a high-pass filter for use in detecting a high-frequency component of the electrical discharge voltage, reference numeral 5 denotes a rectifying circuit for rectifying the high-frequency component from the high-pass filter 4, and an output signal vrec is outputted from the rectifying circuit 5. In addition, reference numeral 6 denotes an electrical discharge generation detecting circuit for detecting generation of the electrical discharge in the machining clearance between the electrode 2 and the workpiece 3. The electrical discharge generation detecting circuit 6 is composed of an electrical discharge voltage detecting circuit 60 for detecting the electrical discharge voltage at the machining clearance between the electrode 2 and the workpiece 3 and of an electrical discharge current detecting circuit 61 for detecting an electrical discharge current at the machining clearance between the electrode 2 and the workpiece 3.
An output signal 60s of the electrical discharge voltage detecting circuit 60 and an output signal 61s of the electrical discharge current detecting circuit 61, in the electrical discharge generation detecting circuit 6, are inputted to a logic circuit 62. Reference numeral 7 denotes a delay circuit. The delay circuit 7 is composed of a time constant circuit 70 for measuring a time constant tH of the high-pass filter 4 and of a logic circuit 72. An output signal 63 from the logic circuit 62 is inputted to the time constant circuit 70 and the logic circuit 72 in the delay circuit 7. An output signal 71 from the time constant circuit 70 is inputted to the logic circuit 72. Reference numeral 8 denotes an integrating circuit. The integrating circuit 8 is composed of an operational amplifier 80, a capacitor Cl connected between the inverting (-) input terminal and the output terminal of the operational amplifier 80, and a resistor R1 connected in series between the output terminal of the rectifying circuit 5 and the inverting (-) input terminal of the operational amplifier 80. In addition, non-inverting (+) input terminal of the operational amplifier 80 is connected to the ground.
Reference numeral 9 denotes a reset circuit. The reset circuit 9 is comprised of a transistor of which collector-emitter terminals are connected between both terminals of the capacitor Cl. An output signal 73 from the logic circuit 72 of the delay circuit 7 is inputted to the reset circuit 9. Then, reference numeral 10 denotes a comparator. An integrated output value Vint as the output signal from the operational amplifier 80 of the integrating circuit 8 is inputted to the inverting (-) input terminal of the comparator 10, and a reference voltage Vref is inputted to the non-inverting (+) input terminal of the comparator circuit 10.
FIG. 12 shows input and output signal waveforms at main parts in FIG. 11. Reference character A in FIG. 12 indicates an electrical discharge voltage waveform at the machining clearance between the electrode 2 and the workpiece 3. Reference character B in FIG. 12 indicates an output signal waveform of the high-pass filter 4. Reference character I in FIG. 12 shows an output signal waveform in the logic circuit 72. Reference character F in FIG. 12 shows an integrated output signal waveform of the integrating circuit 8.
The operation of this arrangement will be described referring to FIGS. 11 and 12.
In FIGS. 11 and 12, reference numeral 20 denotes an electrical discharge voltage waveform at the machining clearance between the electrode 2 and the workpiece 3, wherein Ton denotes an electrical discharge pulse width and Toff denotes a rest time. When an electrical discharge is generated after applying a voltage to the machining clearance between the electrode 2 and the workpiece 3, both the output signals from the electrical discharge voltage detecting circuit 60 and the electrical discharge current detecting circuit 61 become H (high) levels. These output signals are inputted to the logic circuit 62. In the logic circuit 62, when all these input signals become H levels, i.e., when an electrical discharge is generated at the machining clearance between the electrode 2 and the workpiece 3, an L (low) level is outputted. Such a time is defined as an electrical discharge detecting time t1. t2 denotes a time (t2=t1 +tH) after the time constant tH of the high-pass filter 4 from the electrical discharge detecting time t1. Reference numeral 21 denotes a high-frequency component of the electrical discharge voltage. Reference numeral 22 denotes a disturbance waveform caused by a transient characteristic of the high-pass filter 4.
In the time constant circuit 70, the H level is outputted for the time period tH from the fall time of the output signal 63 of the logic circuit 62. The output signal 63 of the logic circuit 62 and the output signal 71 of the time constant circuit 70 are inputted to the logic circuit 72, and then the output signal 73 indicated at the code I in FIG. 12 is outputted. The rise time of the output signal 73 is defined as t2 at I in FIG. 12.
The reset circuit 9 resets the integrating circuit 8 for a period when the output signal 73 of the logic circuit 72 is the H level. That is, the output signal vrec from the rectifying circuit 5 is integrated at the integrating circuit 8 only for a period when the output signal 73 of the logic circuit 72 is the L level. In the comparator 10, the reference voltage Vref and an integration output Vint indicated by F in FIG. 12 are compared with each other. When the integration output Vint is higher than the reference voltage Vref at the end of the electrical discharge pulse width Ton, it judged to be a normal electrical discharge pulse. Otherwise, it is judged to be an abnormal electrical discharge pulse such as an arc electrical discharge pulse.
However, the aforesaid electrical discharge machining machine has some disadvantages as described below.
Referring to FIGS. 13a and 13b, a first disadvantage will be described.
FIGS. 13a and 13b are timing charts respectively showing a relation between an electrical discharge voltage waveform 20 and an integration output value Vint in the case that the same machining current value is selected. FIG. 13a shows a case in which the electrical discharge pulse width is a large one Ton1, and FIG. 13b shows another case in which the electrical discharge pulse width is a small one Ton2.
The integrated output value Vint from the integrating circuit 8 can be expressed by the following equation (1). EQU Vint=vrec.times.Ton/(R1.times.C1) (1)
where, vrec denotes an output signal from the rectifying circuit 5, Ton denotes an electrical discharge pulse width, R1 denotes a resistance value for determining an integration gain of the integrating circuit 8, and C1 denotes an electrostatic capacitance value for determining the integrating gain of the integrating circuit 8. It is known that a magnitude of the high-frequency component of the electrical discharge voltage depends on a magnitude of the machining current. However, as apparent from the equation (1), even if the same machining current value is selected as the machining condition, the integration output value Vint is proportional to the electrical discharge pulse width Ton. As shown by the integration output signal waveforms F in FIGS. 13a and 13b, it is necessary to change and set the reference value Vref in accordance with the electrical discharge pulse width Ton, which is set as a machining condition. The electrical discharge pulse width Ton is set, as the machining condition, to have a wide value ranging from a minimum value of about 2 sec to a maximum value of about 4096 sec. Then, as expressed by the equation (1), since the integration output value Vint is proportional to the electrical discharge pulse width Ton, the reference voltage Vref is also outputted, employing a data table shown in FIG. 14, corresponding to the electrical discharge pulse width Ton. Thus, a fine setting is not provided so that Vref11 is outputted as a reference value in respect to a electrical discharge pulse width of about 1025 to 2048 sec, for example.
In addition, it is possible not to use the electrical discharge pulse width but to always use Ton2, for a time to be compared with the reference value Vref, in case the electrical discharge pulse width is small. However, if the electrical discharge pulse width is large, an S/N ratio of the integrated output value Vint is decreased and that detecting accuracy is lowered.
Then, referring to FIGS. 15, 16 and 17, a second disadvantage will be described. FIG. 15 is a circuit diagram of the machining power source described in Japanese Utility Model Publication No. 57-33949. Reference character B1 denotes a DC power source, B2 denotes an auxiliary power source, S1 denotes a first switch, S2 denotes a second switch, D1 denotes a first diode, D2 denotes a second diode, R2 denotes a current detecting resistor, C2 denotes a capacitor, L1 denotes a reactor, reference numeral 400 denotes a pulse generator, and reference numeral 300 denotes a control circuit for the first switch S1.
FIG. 16 shows input/output signal waveforms at the main parts in FIG. 15. Numerals 1 and 0 of the first switch S1 and the second switch S2 denote ON/OFF states of the switches S1 and S2, respectively, wherein L1 denotes a current waveform flowing in the reactor L1 which is detected by the current detecting resistor R2. The switch S2 is kept in the ON state during a full period of the electrical discharge pulse width Ton. The control circuit 300 controls the switch S1 in such a manner that the output current becomes a predetermined value. As shown in FIG. 16, the switch S1 repeats ON/OFF operations for several times during the period of the electrical discharge pulse Ton. A large amount of electrical power is consumed in the resistor in a system which uses a resistor as a current limiting element. However, the system shown in FIGS. 15 and 16 is a current control system in which the current is controlled by the reactor L1 and the control circuit 300, so that it can be said that this is a superior system in which the electrical power consumed in the circuit is quite low. This type of machining power source is defined as a reactor type power source hereinbelow.
FIG. 17 shows input and output signal waveforms at the main parts when the reactor type power source is used in the electrical discharge state detecting device shown in FIG. 11. Reference character A in FIG. 17 indicates an electrical discharge voltage waveform, reference character B in FIG. 17 indicates an output signal waveform of the high-pass filter 4, reference character I in FIG. 17 indicates an output signal waveform of the logic circuit 72, and reference character F in FIG. 17 indicates an output signal waveform of the integrating circuit 8. Referring to the waveform A in FIG. 17, a spike-like voltage, which is synchronous with the ON/OFF transitions of the switch S1, appears in the electrical discharge voltage waveform 20 as shown by a numeral 23 in addition to the high-frequency component 21. This spike voltage 23 is a high-frequency component which is generated by an operation of the switch S1 without any relation with the electrical discharge machining phenomenon. Erroneous detection is caused by the spike voltage 23 in the electrical discharge state detecting device shown in FIG. 11, which is intended to detect the high-frequency component in the electrical discharge machining phenomenon.
This erroneous detection will be described hereunder.
The component of the spike voltage 23 of the electrical discharge voltage waveform A in FIG. 17 appears in the output signal waveform of the high-pass filter 4, as shown by the waveform B in FIG. 17, as the spike voltage 24. The time duration, in which the spike voltage 24 appears is defined as b1 when the switch S1 is turned on, and defined as b2 when the switch S1 is turned off. On the other hand, the time duration in which this spike voltage 24 appears depends upon the magnitude of the machining current. Reference character F in FIG. 17 shows an output signal waveform of the integrating circuit 8. Its integrated output value increases, as indicated by a solid line, every time the spike voltage 24 is generated. An integrated output value is shown as Vint2 when the reactor type power source is used as the machining power source, while an integrated output value is shown as Vint1 when it is not used. Since the integrated output value Vint2 is a sufficiently larger value than the integrated output value Vint1, it is impossible to perform accurate detection of the high-frequency component due to the electrical discharge phenomenon. That is, erroneous detection occurs due to the electrical discharge pulse, the integrated output value of the high-frequency component in the electrical discharge phenomenon is small and thus should be ordinarily be judged as an abnormal electrical discharge, but is nevertheless judged as a normal pulse.
In view of the above, the present invention is made in order to solve these disadvantages, and it is an object of the present invention to provide an electrical discharge state detecting device for an electrical discharge machine which is capable of correctly detecting a normal electrical discharge pulse in an electrical discharge state of the electrical discharge machining.